A Method for gyrotron beam coupling into a corrugated waveguide

Oda, Yasuhisa; Kajiwara, Ken; Takahashi, Koji; Sakamoto, Keishi

In the radio-frequency (rf) power transmission system of the electron cyclotron heating and current drive (EC H&CD) system, a gyrotron power should couple with a fundamental mode of the corrugated waveguide (HE mode) for a low loss long-distance power transmission and direct Gaussian-like beam radiation from waveguide end. To achieve high HE mode purity, a beam coupling method with the measurement of transmission mode in the waveguide was examined using a 170 GHz high power gyrotron. In beam coupling, the offset and tilt angle of the input beam at waveguide inlet was minimized by controlling angles of mirrors in the matching optical unit (MOU) to minimize unwanted LP mode in the waveguide. According to analyzed mode content, the HE mode content was optimized. After optimization procedure of beam coupling, 95% of HE mode purity in the waveguide was achieved, which is a first demonstration of the criteria of EC H&CD system of ITER.